The present invention generally relates to compression systems and compression and expansion systems for composite video signals, and more particularly to a system in which certain number of points of a video signal within a composite video signal excluding synchronizing signals are extracted, and only the information concerning level and position of the extracted points is compressed and transmitted, and further, parts of the transmitted compressed signal between the extracted points are obtained by use of a connecting linear function, to expand the complete compressed signal, and the synchronizing signals are mixed to the expanded signal to obtain the composite video signal.
Generally, when a signal is to be sampled, and this signal has a band between 0 Hz and W Hz and does not have a frequency component over W Hz, it is known from the sampling theorem that information loss will not be introduced if the signal is sampled with an interval of 1/2W seconds. If the video signal has a band between 0 Hz and approximately 4 MHz, the sampling frequency for sampling this video signal is 2.times.4 MHz, the horizontal scanning period is 6.times.10.sup.-5 seconds, and the lightness (luminance level) is described by eight bits (254 steps), 3840 (2.times.4.times.10.sup.6 .times.6.times.10.sup.-5 .times.8) bits of information will be required during one horizontal scanning period by use of the normal sampling method.
However, the spectrum distribution of the video signal gradually decreases in the high frequencies, and it is unlikely that the entire spectrum will be required. Moreover, the voltage of the video signal corresponds to the lightness. When recognizing a picture, maximum points, minimum points, and rapidly changing points of lightness are generally noticed, but parts between these points where the lightness gradually change, are not noticed. Accordingly, in the normal sampling method, even the information which is hardly noticed is sampled, and there was a disadvantage in that this sampling operation performed with respect to these unnoticed points were not necessary.